1. Field of the Invention
The present invention relates to a magnetostrictive torque sensor to detect a torque applied to a rotating shaft having magnetostrictive characteristics, based on a change in inductance of a detection coil.
2. Description of the Related Art
In power steering mechanism, engine control mechanism, power transmission device etc. for vehicle, it is highly necessary to detect a torque applied to a steering shaft and other passive axes etc. that are rotating shafts.
It is generally known that a material having the magnetostrictive characteristics, for example, Ni, Fe—Al alloy, Fe—Co alloy etc. causes a variation in relative permeability when external force is applied thereto, where the relative magnetic permeability decreases in the compression stress direction and increases in the tensile stress direction.
JP-A-2005-164531 discloses a magnetostrictive torque sensor using this principle.
As shown in FIG. 13, the magnetostrictive torque sensor 110 of JP-A-2005-164531 comprises a rotating shaft 111 having magnetostrictive characteristics, a pair of semi-cylindrical magnetic cores 114, 115 having detection coils attached onto its inner periphery surface, the coils comprising coils 112a, 113a inclined at an angle of +45° to the center axis O of the rotating shaft 111 and coils 112b, 113b inclined at an angle of −45° to the center axis O, and an alternating current signal generating circuit (not shown) for applying an alternating current voltage to the coils disposed in the semi-cylindrical magnetic cores 114, 115.
In the magnetostrictive torque sensor, it is simulated that, as shown in FIG. 14, a torque T is applied to the rotating shaft 1 such that it is produced in the counter-clockwise direction on the left side of the drawing and in the clockwise direction on the right side of the drawing (herein, these directions of the torque T is defined as a positive direction) viewed from the axial direction X. At this time, viewed from the left side of the rotating shaft 111, a compressive stress is applied in +45° direction of the rotating shaft 111, and a tensile stress is applied in −45° direction thereof. Viewed from the right side of the rotating shaft 111, a compressive stress is applied in −45° direction of the rotating shaft 111, and a tensile stress is applied in +45° direction thereof. This principal stress σ is proportional to the torque T, and derived from the following formula, when a diameter of the rotating shaft 111 is determined as D:σ=16T/(πD3)  (1)
If the rotating shaft 111 has the magnetostrictive effect, an axial magnetic anisotropy Ku will be induced by the principal stress σ, and derived from the following formula (2).Ku=2·(3/2)λsσ=48λsT/(πD3)  (2)
wherein λs is a saturation magnetostrictive constant of the rotating shaft 111.
Due to the axial magnetic anisotropy Ku, a +σ direction becomes an easy magnetization direction and a −σ direction becomes a difficult magnetization direction. In connection with the magnetostatic energy, the relative magnetic permeability in the easy magnetization direction, i.e., the +direction increases, and the relative magnetic permeability in the difficult magnetization direction, i.e., the −σ direction decreases to the contrary. Therefore, when current flows into the coils 112b, 113b inclined to the easy magnetization direction (i.e., the tensile stress direction), the relative magnetic permeability in the tensile stress direction increases so that faradic current flows in a direction to decrease the magnetic flux in the tensile stress direction. Then, faradic voltage is induced by the faradic current so that the inductance of the coils 112b, 113b increases. On the other hand, when current flows into the coils 112a, 113a inclined to the difficult magnetization direction (i.e., the compressive stress direction), the relative magnetic permeability in the compressive stress direction decreases so that the inductance of the coils 112a, 113a decreases.
As shown in FIG. 15, a bridged circuit for detecting a change in inductance as described above is composed of four coils 112a, 112b, 113a, and 113b. A terminal d of the coil 112a and a terminal e of the coil 112b are connected. Similarly, a terminal b of the coil 113b and a terminal g of the coil 113a are connected. A high frequency current I generated from an oscillator (i.e., an alternating current signal generating circuit) A flows into the terminal c of the coil 112a and the terminal a of the coil 113b, and flows out from the terminal f of the coil 112b and the terminal h of the coil 112a. 
When a positive torque is applied to the rotating shaft 111, inductance L between the coil 112a and the coil 113a decreases by ΔL and inductance L between the coil 112b and the coil 113b increases by ΔL, so that as shown in the following formula (3), the output from the bridged circuit increases by ΔV in the positive direction.ΔV=2×ωΔL×I  (3)
On the other hand, when a negative torque is applied to the rotating shaft 111, inductance L between the coil 112a and the coil 113a increases by ΔL and inductance L between the coil 112b and the coil 113b decreases by ΔL, so that as shown in the above formula (3), the output from the bridged circuit decreases by ΔV in the negative direction. Accordingly, the change of the torque T applied to the rotating shaft 111 can be detected as a change in voltage.
However, the magnetostrictive torque sensor of JP-A-2005-164531 has the disadvantage that, when the magnetostrictive characteristics of the rotating shaft 111 are not uniform, the detection amount (sensor output) of the coils 112a, 113b and the coils 112b, 113a is changed depending on its rotation angle so that reliability on the torque detection must lower, since in the semi-cylindrical magnetic core 114 the coils 112a, 113b detect the magnetostrictive characteristics in one half-circumference region of the rotating shaft 111 and in the semi-cylindrical magnetic core 115 the coils 112b, 113a detect the magnetostrictive characteristics in the other half-circumference region of the rotating shaft 111.